Prior art biological reaction processes for the treatment of sewage suffer from the disadvantage, to varying degrees, of low efficiency in terms of the amount of resources required and the time taken to process a given volume of sewage.
Attempts to increase the efficiency of known biological reactions processes for the treatment of sewage have focused on various parameters of these types of reaction processes including the means for distribution of respiratory air to the reaction vessel; techniques for the reorientation of the support medium to prevent channelling and to introduce greater flow of air and liquid through the support medium; the nature of the support medium; and methods of controlling the air and liquid delivery rates to the system.
For example, Australian Patent Specification 528,760 describes a process for purifying polluted water by percolating it downwardly through a submerged, fixed granular bed. Oxygenated gas is fed upwardly from an intermediate level of the bed and treated water is discharged from the bottom of the bed. The flow of water to be treated and the flow of the oxygenated gas is adjusted in such a way that specific mathematical relationships are satisfied.
In the process described in the Australian Specification 528,760, the critical parameters for efficient operation are said to be the rate of flow of water to be treated over the granular bed and the volume of oxygen supplied to the microorganisms. Of these two parameters, regulation of the dissolved oxygen content of the water is the key to maintaining the process at its optimum rate.
In order to achieve acceptable results with this process, it is necessary to take the water through several pre- and post-treatment stages including pre-oxidation with ozone and filtration through sand.
The process described in Australian Specification 528,760 is described in an article by Barr K. G. in the journal Water of March 1988. In that process, it is intended that the water flow/oxygenated gas flow mathematical relationships achieve high concentrations of biomass growth on the granular bed to allow high loading rates or low hydraulic retention times.
Prior art processes for the treatment of sewage or polluted water have made a number of assumptions regarding the fundamental nature of the biological reaction process. All these prior art processes, including that of Australian specification 528,760, assumed that there is a requirement for a large mass of microorganisms in order to establish a large biofilm density to achieve increased rates of conversion of nutrients to the desired product.
There has also been an assumption that the nutrient and environmental requirements (such as optimal concentration of dissolved oxygen to all parts of the system) of the microbial species play a significant part in the efficient operation of the process.
The various elements of known biological reaction processes have all received investigation on the basis of the desirability for a large amount of biofilm which is thought to result in the highest rates of conversion and to efficient operation.
In particular, prior art processes have, in the main, tended towards finding ways of increasing biofilm quantity.
The improved processes arising from those investigations have met with varying degrees of success such that the more successful processes exhibit high efficiencies in the initial stages of operation only.
However, the nature of the improvements to date have been such that these prior art processes have not been able to maintain the high level of efficiency for prolonged periods. The efficiency gradually falls to unacceptable levels at which point the plant is shutdown in order that backwashing, cleaning, maintenance and other steps may be taken to restore the process to its initial high efficiency.
The reason for the fall in efficiency is thought to be due to the excessively large quantity of biofilm or thick biofilm created by the process which clogs the system and leads to deleterious channelling effects in the process vessel or bioreactor.
Research by the applicant has surprisingly shown that the uninterrupted growth of thick biofilms over long periods of time do not lead to the highest conversion rates but to greater inefficiencies in biological reaction processes.
It has not been found by the present inventors that the highest conversion rate of nutrients and the highly efficient operation of the processes of the invention can be achieved through interrupting the growth of biofilm more frequently than is carried out in the prior art, and particularly in AU 528,760 and that this high efficiency can be maintained for an equally large mass of biofilm as long as the particles chosen for the support medium are of an optimal size.
An object of the invention is to keep the rate of mass transfer of nutrients to the microorganisms as high as possible. This is achieved largely by minimising the detrimental effects of channelling of nutrient and air through the process vessel or bioreactor.
The mass transfer system of the invention involves the movement of dissolved oxygen and other nutrients from the liquid feed to the biofilm and the removal of products of the biological reaction process. This process can be defined as follows: EQU Nutrients+Cell Mass (microorganisms)=Cell mass +CO.sub.2 +H.sub.2 O+metabolites (products)
A high rate of mass transfer of nutrients is achieved by providing periodic pulses of air through the bioreactor to disrupt the bed of support particles and remove any localized clogging of solid matter in the bed that may lead to channelling of nutrient away from metabolically potent cell mass.
The periodic pulses are preferably of an explosive nature sufficient to disrupt the bed of support particles.
The achievement of a high rate of mass transfer of nutrients is assisted by providing support particles in the bed of an optimal size. The smaller the effective size of the support particles, the greater is the total bed surface area available for attachment and growth of the microorganisms and, accordingly, the greater the surface area of resultant biofilm. However, as the accumulation of smaller support particles create smaller gaps between particles than would the accumulation of larger support particles, the possibility of the biofilm growing to form a bridge across adjacent small sized particles is heightened, with the effect that effective surface area of metabolically potent cell mass falls. The more frequently periodic pulsing with air through the process vessel in the present invention removes these bridges and restores the high effective surface area of metabolically potent cell mass.
In addition, the pulsing occurs at a frequency so as to control the residence time of solids in the bed.